Preheat oven to 180°C. Toast nuts & seeds 5–6 min. Reserve 1/3 pine nuts for topping. Blitz the rest with chestnut purée, suet and syrup until it comes together. Scrape the sides as you go. With lots of flour, roll out the dough on baking parchment as thinly as possible (less than 5mm). Transfer to large shallow baking tin, prick with fork, cover with parchment and bake blind (e.g., filled with dry chickpeas) 12-15minutes.
Meanwhile, make the topping. Heat 1tbsp oil over a medium heat and sauté mushrooms 6–8 minutes to golden. Set aside. In the same pain, heat 1tbsp oil and wilt spinach (or heat through if frozen). Blitz spinach, avocado, tofu, pepper, nutmeg and lemon juice to smooth. Thinly slice sweet potato with a peeler, and fry over high heat 4-5min to just crisp.
Assemble: Spread spinach over base, then top with mushrooms, sweet potato slivers and a extra toasted pine nuts.
Eat heated or at room temperature, with potatoes and vegetables.